Conduction of the cardiac action potential is influenced by electronic interactions between heart cells. This electrical communication is mediated by the presence of intercellular channels, referred to as gap junctions, in the sarcolemma of adjoining cells. We will record the activity of individual gap junction channels in pairs of embryonic chick ventricular and rabbit mammalian sinus node cell using the double whole- cell patch clamp technique. Measurements of macroscopic and single channel junctional conductances, coupling coefficients, and input resistance measurements will be made to determine the cellular mechanisms involved in the regulation of electrical communication within the sinus node by transjunctional voltage and acetylcholine. Possible mechanisms of action include effects on gap junction channel gating, channel conductance, or nonjunctional membrane resistance. In other investigations, the two- dimensional distribution of gap junction proteins will be examined to determine if the abundance of gap junctions correlates directly with preferential sinus node-atrial conduction pathways. The cellular morphology of the sinus node junctional plagues will also be examined using freeze-fracture and immunolocalization electron microscopy. Using chick ventricular cell, pairs we will the hypothesis that several of the class I antiarrhythmic drugs (e.g., lidocaine and procainamide) have effects of electrical communication in addition to their sodium channel blocking activity. We will distinguish between mechanisms involving direct effects on gap junction channels, secondary effects on gap junctions via alterations in intracellular sodium and calcium activities, or direct effects of nonjunctional membrane resistance. Junctional conductance, coupling coefficient and input resistance measurements will again be employed to determine the cellular mechanisms involved. We will determine the drug concentrations necessary to produce the effect on electrical coupling and relate these effects to their therapeutic action in the heart. Our long term objective is to improve our understanding of how cardiac gap junctional conductance is regulated and how this relates to normal and abnormal conduction of the cardiac action potential.